Carbon Dioxide Systems

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Carbon Dioxide Systems are the foundation of your tank. Here is a detailed look at a CO2 system.

This article is contributed by Aqarium Advice member Madasafish

Introduction – Carbon Dioxide (CO2) connects the respiratory processes of the animal and plant kingdoms. It is one of the main products of respiration by animals and plants (as they respire during the night), and it provides the carbon necessary for plants to create organic matter and oxygen out of simple compounds. CO2 systems pose one of the greatest challenges of money, time, and ability to the freshwater aquarist. Undoubtedly, though, they promise astounding results. Takashi Amano and other renowned aquarists have created breathtaking aquascapes through pressurized CO2 systems.

But why are CO2 systems so important to aquarists interested in having planted tanks? Because of a relatively simple scientific formula (bear with me here!): CO2 + E + H2O +… => O2 + C6H12O6 + H2O +…

This unbalanced formula outlines the general (but not complete) reaction involved in photosynthesis. Plants create oxygen (O2) and sugars (C6H12O6) for growth during the day by absorbing carbon dioxide (CO2), water (H2O) and light energy (E) along with important nutrients such as iron, nitrate, phosphate and potassium. In an aquarium with standard lighting and no CO2 system, a small amount of CO2 is naturally present in the water. It is present because it interacts with the atmospheric CO2 at the water’s surface, and it is present because fish breathe out (respire) CO2.

In such a setting, some plants such as Anubias species (spp.), Java Fern and Java Moss grow quite well. This is because they require only low levels of light to transform (photosynthesize) CO2 into O2. However, most plants, such as baby tears, Glossostigma elatinoides and Ludwigia spp., require relatively high light to perform this same transformation. As the equation indicates, with higher light, more water and carbon dioxide are required for the plants to photosynthesize, grow, and indeed live! So, in an aquarium with strong lighting (more than 2 watts per gallon), carbon dioxide must be added to the water in order to balance the above equation.

Balancing CO2 in an aquarium is no easy task, but it can be achieved with various levels of commitment. Below are a few sections on these levels (i.e. how much money, time and ability you’ll have to commit to the set-up), and other paragraphs on the various elements essential to a CO2 set-up. Often, the most expensive setups provide the best results and require the least amount of effort after they are set up these are the advantages of a greater investment. May your planted tank flourish!

JBJ Regulator with Solenoid and Bubble Counter

JBJ Solenoid Valve

JBJ Bubble Counter

Aquaclear Water Pump (left), tubing and Aquamedic Reactor (right, partially obscured). The CO2 tubing line is black, and can be seen at the upper right-hand portion of the image, connected to the reactor.

Milwaukee pH Regulator

Solenoid Regulator with Solenoid Cable Connected

Cabomba aquatica

Low-Cost and DIY Setups – The cheapest commercially available setups for small tanks are available through Hagen. The Hagen CO2 Natural System usually retails for $22-$35. The devices are appropriate for small tanks (5-20 gallons) with low-to-moderate lighting, as they provide a moderate amount of relatively unregulated CO2. They should not be trusted, however, to provide adequate CO2 to demanding, high-light plants.

The device combines sugar, a small amount of baking soda, and yeast. The yeast consumes (ferments) the sugar, creating CO2. The baking soda prevents the natural acidification of the solution by the CO2, and keeps the fungi (yeast) alive for longer. The CO2 created by this small biological system bubbles up through a tube at the top of the unit, and enters the aquarium, where is released into the water. In order to ensure that the water absorbs as much CO2 as possible, the bubbles are released onto a ladder, a small plastic diffusing device secured to the interior wall of the tank. They rise up the ladder and eventually escape to the water’s surface. The sugar/yeast/baking soda/water mixture will have to be replaced every 2-4 weeks.
COST: $22-35

Because a simple CO2 system involves few components, many hobbyists choose to design their own setups based on others designs. These setups typically include an emptied 2L bottle of soda, some air tubing, and some sort of homemade diffuser. The bottle of soda is typically filled about 1/5 to 1/4 full of sugar, a tablespoon of baking soda and a small pinch of yeast, and then filled to about 90% full with old, cycled water. A very small hole is bored through the cap of the soda bottle, preferably with a 7/32, 3/16 or 1/4″ drill bit, and the 1/4 tubing is inserted as tightly as possible into this hole. The tubing should then be glued to the cap, and the cap should be screwed tightly onto the bottle in order to prevent leaks. If the system leaks, the amount of pressure to the diffuser/reactor in the tank decreases, and CO2 is lost.

Bare COST: $5-20
COST with necessary components and test kits: $25-80

Pressurized Systems – Undoubtedly, pressurized systems are more expensive than the systems just mentioned. However, they are absolutely necessary for larger tanks and higher-light plants, and are ultimately more easily controlled, and more autonomous.

a. A pressurized system starts with a CO2 cylinder. These can be bought online for anywhere from $30 to $200. Recommended sizes include 5 lb, 10 lb and 20 lb cylinders, as these will give a 55-gallon tank adequate CO2 for many months. Keep in mind that aluminum cylinders are lighter than steel cylinders, and that many cylinders sold on Ebay must be tested for safety before being filled with CO2. This test will usually cost you about $25, and a fill-up will cost you anywhere from $10 to $30. Many people fear that CO2 cylinders will leak and cause harm to a house’s inhabitants. Rest assured that CO2 (unlike CO, Carbon Monoxide) is one of the least reactive and least toxic gases known to man. If it does leak, CO2 levels may increase in a house, but will almost certainly not rise to a toxic level (it happened to me!).
COST: $70-200

b. A CO2 cylinder, when opened, will vent a large amount of CO2 very quickly. Accordingly, it must be regulated to provide a slow, constant flow of CO2. The device designed to control this flow is called, rather appropriately, a CO2 regulator. It usually consists of a) an connector that seals the cylinder to the regulator, b) a dial that indicates the pressure inside the CO2 cylinder, c) a second dial that indicates the pressure of the released gas, d) a needle valve dial that allows you to regulate the amount of gas being released from the cylinder, and e) a 1/4-inch output swagelok ® tube and nut where 1/4-inch (standard) tubing can be connected to the device. Typically, a CO2 cylinder will have an interior pressure on the order of 1000s of pounds per square inch (psi), and the escaping gas will have a pressure of 10s of psi.
COST: $60-150

c. A good regulator setup, such as the Milwaukee All-In-One Regulator or the JBJ Regulator, will also offer a solenoid. A solenoid is a small device that opens a valve when a current of electricity is run through it, and which closes the same valve when the current is stopped. When put on a simple timer, the solenoid will start and stop the flow of CO2 when you wish it to. Why is this desirable? During the night, most plants stop photosynthesizing, burn sugars and consume oxygen in order to grow. While they do this, they do not need very much CO2. CO2 pumped into a tank after the lights have turned off may reduce the pH of the tank (see the section of CO2 chemistry), and threaten the life of your fish. Thus, a solenoid attached to a timer provides a welcome way of avoiding having to turn the flow of CO2 off every night, and on every morning manually. A solenoid attached to a pH regulator will keep the level of CO2 in the tank, and the pH of the tank, even more constant.
COST: $50-70

d. A good regulator setup will also include a bubble counter. This device is see-through, contains water, and allows the CO2 to pass through the water on its way to the fish tank. This enables the hobbyist to observe how much air the regulator is releasing at a bubble-by-bubble rate. Most bubble counters are simply screwed onto the out-tube on a regulator, and then attach, above them, to the tubing going to the fish tank. The tubing should be 1/4 inch in diameter, as most regulator and reactor setups deal with this particular gauge of tubing.

e. A reactor/diffuser is necessary in order to take advantage of the CO2 bubbles reaching the fish tank, and can be used in pressurized or non-pressurized systems. Unrestrained, the bubbles will ascend quickly to the surface of the water, and the majority of the CO2 will be lost. Various devices attempt to 1) keep the CO2 in the tank as long as possible, or 2) break up the CO2 bubbles into smaller bubbles in order increase the surface area of CO2 reacting with the water. Most reactors/diffusers require a water pump to push the water and CO2 through the unit. Others are plumbed into the outflow of a canister filter, or rely on the natural buoyancy of gas.

Method 1. The ladder mentioned in section 2 is a simple type of reactor that performs method 1 without a water pump it simply increases the amount of time the bubbles remain in the tank. A bell reactor is an even more basic version of the same. It simply holds the gas as long as possible in an inverted bell-like glass reactor. The bell has a small inlet for CO2, and large outlet for the CO2 and water to leave. A more complicated version of method 1 can be found with the Aqua Medic Aqualine CO2 reactor, an interior reactor with long, circular ramps for the CO2 to move up. The Aqua Medic reactor requires a water pump.

Method 2. The deluxe external CO2 reactor from Eheim follows method 2 by using bioballs to break up bubbles of CO2. It is generally plumbed into the outflow of a canister filter, as are most method 2 reactors/diffusers. Most DIY reactors follow this external design.
COST: $10-100

f. Pressurized systems allow you one further way to stabilize and optimize your planted tank the pH regulator, or controller. A pH regulator is a smallish, relatively expensive, device with several crucial elements: a pH probe to measure the pH of the aquarium, a power supply, the regulator box where the desired pH level is set, calibrated and can be monitored, and the solenoid-regulator. Because CO2 added to water decreases the pH of the water, the pH regulator is able to turn on and off the solenoid allowing or restricting CO2 flow into your aquarium whenever it finds the pH of the aquarium to be too high or low enough.

The device works in the following manner. The main unit (regulator box) is plugged in, and a desired pH is set on the dial. The probe is then inserted into the aquarium, and connected up to the regulator box. The solenoid regulator (which resembles an AC adaptor with holes on the top for another unit to be plugged into it) is plugged into the power strip, and the power cord for the solenoid is plugged into it. When the regulator box has registered the pH of the aquarium, it acts accordingly. If the pH of the aquarium is lower than the desired level, the solenoid is shut off, stopping the flow of CO2 to the aquarium. When the pH increases to slightly above the desired pH level, the solenoid is turned on, and the CO2 from the CO2 regulator starts to flow out. The addition of the CO2 to the water lowers to pH of the water, and when the CO2 has lowered the pH to the desired level, it shuts the solenoid off. It turns on and off accordingly throughout the day and night.

COST: $5-20

To avoid quick pH swings, it is best to try and manually regulate the rate of CO2 coming out of the CO2 regulator (by turning the needle valve up or down and watching the number of bubbles that come through the bubble counter) to a point that keeps the pH more or less stable. I find that with my 55-gallon tank setup, I can keep the pH more or less stable with a CO2 bubble count of 2-3 bubbles per second.
COST: $70-200+

Water Chemistry – CO2 water chemistry is enormously complicated! Geochemists are still trying to understand the fine balance of CO2 between the atmosphere and oceans (and lakes/rivers). But, for aquarists purposes, we must remember only a few crucial points about the interaction of CO2 with water.

#1. As CO2 levels increase in water, so does the acidity of the water. In other words, higher CO2 levels usually create lower pH levels (e.g. 6, 5). pH is an (exponential) measure of the number of H+ ions in a solution. The more H+ ions there are, the lower the pH.

This is a rough equation to represent the balance of these elements. (The double arrow in the middle means that the equation can be pushed to either side by adding more components to one side. So, if CO2 is added, the equation will move right, creating more H+ and HCO3-; if more H+ ions are added, the equation will move to the left, creating more H2O and CO2.)

H2O + CO2H+ and HCO3– Because pH is a measure of the number of H+ ions in a solution, increasing levels of CO2 and keeping the amount of water constant will push the equation above to the right, creating more H+ ions, decreasing the pH! Reducing the amount of CO2 will have the opposite effect, and will force the equation to the left, increasing pH.

#2. CO2 levels in a planted tank must be maintained between 10 ppm (parts per million) and 25 ppm.
10 ppm is the lowest level ideal for plants, but 25 ppm is not the highest possible ideal level! It is simply a cap imposed for the health of one’s fish. If levels creep higher than 25 or 35 ppm, fish have a hard time breathing, and their health is threatened.

#3. KH and pH tests allow you to measure the CO2 level in your planted tank.
An even more complicated equation than that mentioned in #1 factors in the influence of KH on pH and CO2 levels. We’ll keep it simple by saying this. If no phosphate buffers are present in the tank (these are present in products use to buffer the pH of aquariums, and must not be used in planted tanks), KH and pH levels can be used to calculate the level of CO2 in the water. The link for this calculation is:
According to the table provided, a KH of 3 degrees is ideal if you have a pH of 6.6-6.8, and a KH of 6 is ideal for a tank with a pH of 7-7.2.

How do you measure this? Simple test-tube reagent kits can be bought at most stores, or online, to measure pH and KH. pH probes can also be bought for $30-60 to measure pH more quickly. When you have measured the KH of your water, try to find the corresponding pH level ideal for safe CO2 levels on the chart provided in the link. A pH regulator will create this pH level most simply. Daily testing of the water, and a determination of how fast to allow CO2 to flow from your CO2 cylinder will do the same if you do not have a pH regulator, but will require more work! You can increase the KH of your water by adding crushed coral or limestone to the tank. Small amounts of baking soda (NOT baking powder) will do the same thing on a more temporary basis.

Tips – Most success with planted tanks hinges on knowing some of the important ins and outs of the equipment. So, this section will address a few important tips pertinent to CO2 that will help you create a successful and beautiful planted tank.

a. Surface agitation removes CO2 from water. The level of CO2 in the water after CO2 injection is unnaturally high, and agitation of the water’s surface encourages the CO2 to escape, returning the water and air to the natural dissolved-gas equilibrium state. You can combat this by using filters that do not disturb the water’s surface. A canister filter is ideal, as it usually takes and returns water from below the surface of the tank. Oxygen deprivation, which usually comes up in the context of aquariums, is overcome by the oxygen the plants create. If, however, the plant cover in the tank is minimal, you may want to add an air stone to the tank to add oxygen for the fish. There is no problem with a hang-on-the-back filter, though you will find that have to inject CO2 at a faster rate to keep up with the loss of CO2 at the water’s surface.

b. Phosphate buffers, mentioned above, should never be used in the context of planted aquariums. The pH/KH monitoring system is so crucial to understanding the level of CO2 in the aquarium, that it cannot possibly be dispensed with. Use crushed coral or limestone, instead, to buffer the water’s pH up. These are calcium carbonate, and will add to the amount of carbonate (KH) in the tank. This will not interfere with the pH/KH system of testing for CO2 levels.

c. A good indicator that CO2 levels in the tank are not sufficient is great algae growth. Algae tend to flourish in low-CO2 situations, and is usually reduced or eliminated with CO2 levels over 20 ppm. If you are having a lot of algae problems, it’s quite likely that you need to increase the bubble rate of CO2 to your tank.

d. Another good indicator that CO2 levels in your aquarium are not sufficient is that your plants are becoming see-through and yellow. When plants have a lot of light, but not enough CO2, they tend to try to grow nonetheless. In this case, they miss out on essential organic carbon, and their leaf structure becomes compromised. Without CO2 injection, these leaves will eventually die and fall off. A healthy plant with good CO2 levels may pearl if its leaves are thin enough–small bubbles of oxygen will accumulate on or at the tip of the leaves. A healthy plant with high light levels may also turn red at the top. The Cabomba below is pearling and turning slightly red.

Shopping List

Soda Bottle: $0
3-8 feet of tubing: $1-3
Sugar: $0.25-0.50 per dose
Yeast: $0.50-$3.00
Baking Soda: $0.10 per dose
(Optional) Bubble stone: $1-2
Commercial Reactor: $10-50
pH Test Kit: $6-10
KH Test Kit: $6-10
TOTAL: ~$25-80

Hagen CO2 Natural System: $22-35
Commercial Reactor: $10-50
(Optional) Bubble Stone: $1-2
pH Test Kit: $6-10
KH Test Kit: $6-10
TOTAL: ~$45-$107

10-lb CO2 tank: $70-130
Milwaukee All-in-One: $70-120
6-ft length of tubing: $1-3
pH Regulator (recommended): $70-200+
Commercial Reactor: $10-50
pH Test Kit: $6-10
KH Test Kit: $6-10
TOTAL: ~$230-620+

Filed under Articles, Freshwater.